Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
Receivers include a number of components that are arranged to convert a received wireless signal, usually in the radio-frequency spectrum, to a digital signal capable of being processed by digital circuitry (e.g., microprocessors, digital signal processors, etc.). Selected components of a traditional receiver are shown in FIG. 5. As shown in FIG. 5, a traditional receiver 21 may include a low-noise amplifier (LNA) 34 to amplify a received radio-frequency (RF) signal. The amplified RF signal may then be frequency downconverted (i.e., demodulated) based on an oscillation signal produced by oscillator 10. The downconverted signal may then be filtered by low-pass filter 36, providing a low-pass filtered analog signal at its output. This analog signal may be converted to a digital signal by analog-to-digital converter (ADC) 24. Such digital signal may then be communicated to digital circuitry for further processing.
In such traditional receiver designs, the analog signal received by ADC 24 is in the form of a voltage. This voltage is usually converted into a current using a resistor or transconductor within ADC 24. Therefore, low-pass filter 36 must avoid any signal clipping. Low-pass filter 36 must provide sufficient filtering of undesired signals and exercise automatic gain control over a large range. Low-pass filter 36 must also satisfy a specific set of in-band characteristics such as group-delay variation and droop. All of these requirements for low-pass filter 36 render it costly, power-hungry, and complex.
In addition, analog-to-digital converters must typically satisfy blocking specifications. Blocking specifications may refer to the capability of the receiver to withstand extremely powerful signals at a frequency other than the desired signal (e.g., at a frequency of 20 KHz more or less from the frequency of the desired signal). If the input of a receiver receives such powerful undesired signals in close frequency proximity to the desired signal, the unwanted signals may cause the receiver to become insensitive to the desired signal.
Furthermore, a conventional receiver, such as that depicted in FIG. 5, radio frequency and analog blocks preceding ADC 24 may generate a direct current (DC) voltage, or slowly varying, signal known as a DC offset. In such a receiver, baseband blocks after downconverter 28 may provide a large DC gain to amplify the desired signal. The adverse effect of this large DC gain is amplification of the DC offset. This leads to a large DC voltage at the input of ADC 24 which can severely limit its dynamic range. The conventional solution to the DC offset problem includes filtering the output signal of ADC 24 a digital filter in order to measure DC offset at the input of ADC 24. The digital code generated by the digital lowpass filter is subsequently applied to a digital-to-analog converter (DAC). The DAC output is roughly equal to the DC offset and it is fed back into the ADC in order to cancel the DC offset. This DC offset cancellation (DCOC) loop requires extra hardware (digital filter and DAC) which adds to the complexity and cost of the system. Furthermore, DCOC is performed multiple times across different settings for a multi-mode receiver, which may lead to long calibration time.